Multiple field cassegrain-type optical combination

ABSTRACT

The invention is directed to multispectral multifield optical combinations that are intended to be integrated into an optical device. A Cassegrain-type multifield optical combination has an image focal plane, comprising a first holed concave primary mirror that is fixed relative to the image focal plane and a convex secondary mirror.

FIELD OF THE INVENTION

The invention relates to the field of multispectral multifield opticalcombinations intended to be integrated into an optical device,especially into an imaging device.

Imaging devices are used in airborne, naval or terrestrial applicationsin which the imaging devices carry out, for example target detection,recognition and identification functions, target tracking and locatingfunctions and laser illumination and laser countermeasure functions.

DESCRIPTION OF PRIOR ART

The imaging device comprises an input optic that is advantageouslyformed, wholly or partly, by the multispectral multifield opticalcombination forming the subject matter of the invention. Said opticalcombination preferably relates to the input optic of one or more lightsignal receive channels. The number of receive channels is, for example,at least three, these having, as spectral sensitivity range, the nearinfrared or infrared band I, the mid-infrared or infrared band II andthe far infrared or infrared band III respectively. The three receivechannels are therefore isolated from one another, for example usingspectral splitters. Preferably, the near-infrared channel is a multitaskchannel, which includes telemetry, laser spot detection, passive imagingand active imaging, whereas the mid-infrared and far infrared channelsare dedicated to passive imaging.

A multispectral multifield optical combination generally comprises atleast two fields and generally covers at least two spectral sensitivityranges. The various fields have different functions, for example thewidest field is used for target acquisition whereas the narrowest fieldis used to identify and/or track said target. The spectral sensitivityranges of the multispectral multifield optical combination are, forexample, the range from the visible on the one hand to the infrared onthe other.

A multispectral multifield optical combination has several drawbacks,which include, when the spectral range is spread and when the number ofdifferent fields increases, considerable chromatic dispersion combinedwith a bulky and complex architecture. Two types of optical combinationcan a priori be envisioned, namely dioptric or refractive opticalcombinations, that is to say those based on lenses, and catadioptric orreflective optical combinations (that is to say those based on mirrors).A refractive multispectral optical combination usually exhibits verysubstantial chromatic dispersion. A catadioptric multifield opticalcombination usually has a high volume and a high complexity.

According to first prior art, it is known to produce a multispectralmultifield optical combination using mirrors or combinations of the zoomtype or of the bifocal system type that are located in each of thereceive channels. The multispectral multifield optical combinationobtained has the drawback of being very bulky and very complex.

According to second prior art described in patent application FR98/06894, it is known to produce a multispectral multifield opticalcombination using a Cassegrain-type setup. The setup proposed has adrawback of being only a two-field setup, that is to say it is limitedto two fields, which is insufficient for certain imaging devices.

The invention proposes a multispectral multifield optical combinationwhich is a three-field combination, is relatively compact and thechromatic dispersion of which remains relatively low. The multispectralmultifield optical combination according to the invention adopts theCassegrain-type setup of the second prior art, to which is added a thirdfield called “wide field” that is obtained by the addition of awide-field focusing optic and by the particular modification of the pathof the light intended to be focused before it is optionally taken up bya transport optic. Unless otherwise indicated, there is no differencebetween “light” and “light signal”. In the wide-field configuration,said light passes via the wide-field focusing optic without passing viathe mirrors of the Cassegrain setup. To pass into the wide-fieldconfiguration from the medium-field configuration, all that is requiredis to move, from among the mirrors of the Cassegrain setup, just thesecondary mirror. The invention also proposes a method of changing fieldin a multispectral multifield optical combination.

SUMMARY OF THE INVENTION

The invention provides a Cassegrain-type multifield optical combinationhaving an image focal plane, comprising a first holed concave primarymirror that is fixed relative to the image focal plane and a convexsecondary mirror, the structure and the position, in narrow-fieldconfiguration, of the mirrors being such that the light that emanatesfrom an external scene considered as being located at infinity and isintended to be focused in the image focal plane is meanwhile reflectedfirstly by the first primary mirror and then by the secondary mirror,which also includes a second holed concave primary mirror that can moverelative to the image focal plane, the structure and the position, inmedium-field configuration, since the medium field is wider than thenarrow field, of the mirrors being such that the light that emanatesfrom an external scene considered as being located at infinity and isintended to be focused in the image focal plane is meanwhile reflectedfirstly by the second primary mirror and then by the secondary mirror,characterized in that the secondary mirror can move relative to theimage focal plane, and in that the optical combination also includes awide-field focusing optic that can move relative to the image focalplane, the structure and the position, in wide-field configuration,since the wide field is larger than the medium field, of the mirrors andof the wide-field focusing optic being such that the light that emanatesfrom an external scene considered as being located at infinity and isintended to be focused in the image focal plane by means of thewide-field focusing optic is not meanwhile reflected by any of saidprimary or secondary mirrors.

The invention also provides a method of changing field in aCassegrain-type multifield optical combination comprising a narrow-fieldprimary mirror, a medium-field primary mirror and a secondary mirror,comprising, when passing from a narrow-field configuration to amedium-field configuration, a step of moving the medium-field primarymirror so that the medium-field primary mirror blocks the lightreflected by the narrow-field primary mirror in the direction of thesecondary mirror, characterized in that the method also includes, duringpassage from a medium-field configuration to a wide-field configuration,a step of moving the secondary mirror and of moving a wide-fieldfocusing optic so that the wide-field focusing optic focuses the lightcoming from the outside without it being reflected by one of the primaryor secondary mirrors of the optical combination.

The medium field is both narrower than the wide field and wider than thenarrow field. The Cassegrain setup allows the architecture to be readilymade multispectral since the mirrors, for example made of aluminum,exhibit very little chromatic dispersion. The optical combinationaccording to the invention could also be used within a single spectralsensitivity range, but it would lose its benefit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood and other features andadvantages will become apparent from the following description and theappended drawings, given by way of examples, in which:

FIG. 1 shows schematically an example of a multispectral multifieldoptical combination according to the invention in the narrow-fieldconfiguration;

FIG. 2 shows schematically an example of a multispectral multifieldoptical combination according to the invention in the medium-fieldconfiguration;

FIG. 3 shows schematically an example of an embodiment of amultispectral multifield optical combination according to the inventionin the wide-field configuration; and

FIG. 4 shows schematically an example of another embodiment of amultispectral multifield optical combination according to the inventionin the wide-field configuration.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Preferably, in the optical combination according to the invention, theimage focal plane is an intermediate image focal plane. The opticalcombination therefore also has an exit image focal plane, locateddownstream of the intermediate focal plane, the upstream-downstreamdirection corresponding to the direction of propagation of the lightarriving from the outside, to be focused, that is to say from the leftto right in all the FIGS. 1 to 4. To transport the image of theintermediate focal plane to the exit image focal plane, the opticalcombination includes a transport optic located between the intermediateimage focal plane and the exit image focal plane. Said transport opticconsists of two holed transport mirrors, that are fixed relative to theintermediate image focal plane and lie between the intermediate imagefocal plane and the exit image focal plane and are structured andarranged so that the light that emanates from an image located in theintermediate image focal plane and is intended to form an image in theexit image focal plane is meanwhile firstly reflected by the transportmirror closest to the exit image focal plane, since the said mirror isconcave, and then reflected by the transport mirror closest to theintermediate image focal plane. The light signals of the variousspectral bands corresponding to the detectors of the various receivechannels are split just downstream of the transport optic; the opticalcombination therefore preferably has not additional transport opticafter the splitting of the various spectral bands. The presence of atransport optic has the advantage of giving the optical combination alarge mechanical extension between the rear of the primary mirror of thenarrow-field configuration and the exit focal plane, thus making it easyto position the spectral splitting elements without requiring additionaltransport optics. The transport optic is preferably structured andplaced in such a way that the exit pupil of the optical combination islocated near the transport mirror closest to the intermediate focalplane, said exit pupil advantageously lying inside the transport optic,that is to say between the two mirrors that constitute the transportoptic. However, this focusing in the exit image focal plane carried outdirectly after spectral splitting without the use of an additionaltransport optic has the drawback of making pupil conjugation on the coldscreen of the infrared detectors of the various receive channelsdifficult, and this produces a flux of parasitic structure which isadded to the flux of parasitic structure resulting from the centralblocking due to the Cassegrain setup. It may therefore be necessary toadd means for reducing flux of parasitic structure, which has thedrawback of increasing the complexity of the optical combination but toa lesser extent than if additional transport optics were to be addedafter the spectral splitting. Among conventional means of reducing fluxof parasitic structure, mention may be made by way of examples of thefitting of cold or low-emissivity baffles of the oriented mirror type ormicrotrihedral surfaces, or else the use of materials with a negativeluminescence.

To pass from a medium-field configuration to a wide-field configuration,one example of a method of changing field according to the inventioncomprises a step of moving the secondary mirror and the wide-fieldfocusing optic. Preferably, this single step is sufficient for passinginto the wide-field configuration. Said movement step consists of asimultaneous linear translation of the secondary mirror and thewide-field focusing optic. To do this, the optical combination accordingto the invention includes a system for the simultaneous lineartranslation of the secondary mirror and of the wide-field focusingoptic. The secondary mirror and the wide-field focusing optic aretherefore advantageously integral. The translation is preferablyperformed along the optical axis of the optical combination until thefocal plane of the wide-field focusing optic coincides with the imagefocal plane closest to the secondary mirror, that is to say theintermediate image focal plane when the optical combination has twoimage focal planes. The imbalance is preferably compensated for by meansof an imbalance compensation mechanism, this mechanism having, however,the drawback of being relatively complex.

In a preferred embodiment of an optical combination according to theinvention of the type comprising a translational movement of thesecondary mirror and of the wide-field focusing optic, the secondarymirror consists of a metallization of the peripheral part of the rearface of the final optical element of the wide-field focusing optic.Thus, in the case of a purely refractive wide-field focusing optic, thecentral part of the final lens of the wide-field focusing optic servesas a field lens in the wide-field configuration, whereas the peripheralpart of said final lens serves as secondary mirror in both thenarrow-field configuration and the wide-field configuration.

To pass from a medium-field configuration to a wide-field configuration,one example of a method of changing field according to the inventioncomprises a step of moving the secondary mirror and the wide-fieldfocusing optic. Preferably, this single step is sufficient to pass intothe wide-field configuration. Said movement step consists of a rotationthat brings the wide-field focusing optic into position, whileretracting the secondary mirror. To do this, the optical combinationincludes a rotation system for bringing the wide-field focusing opticinto position while retracting the secondary mirror. The secondarymirror and the wide-field focusing optic are advantageously mounted on astructure that can rotate and that includes one or more weights forcompensating for the imbalance. The imbalance compensation mechanismhere is much simpler insofar as it is limited to a few weights placed onthe structure comprising the secondary mirror and the wide-fieldfocusing optic, said weights thus making it possible to balance saidstructure, which remains balanced throughout the rotation movement. Thestructure, the secondary mirror, the wide-field focusing optic and theweight or weights are all integral with one another. Thus, a simplerotation movement allows the secondary mirror to be retracted in favorof the wide-field focusing optic, without the need for a complementaryimbalance-compensating mechanism.

In the optical combination according to the invention, the wide-fieldfocusing optic is preferably purely refractive. Thus, the wide-fieldfocusing optic is very compact. The wide-field focusing optic, althoughpurely refractive, is not excessively dispersive, since themagnification in the wide-field configuration is not very high, andthere is therefore less “image separation” than in the medium-field ornarrow-field configurations. When the wide-field focusing optic is inposition in the optical combination in the wide-field configuration, theimage focal plane of the wide-field focusing optic coincides with thefirst image focal plane encountered downstream of the secondary mirrorin the other configurations, this first image focal plane being theintermediate image focal plane when the optical combination has twofocal planes.

Preferably, the advantageously purely refractive wide-field focusingoptic has a short focal length and a small diameter, its focal lengthbeing smaller than the distance between, on the one hand, theadvantageously holed central part of the secondary mirror in thenarrow-field or medium-field configuration and, on the other hand, theimage focal plane closest to the secondary mirror, its diameter beingsmaller than the external diameter of the secondary mirror. Thewide-field focusing optic is advantageously a multispectral optic, thespectral sensitivity range of which extends from the visible into thefar infrared. Thanks to its short focal length and its small diameter,this multispectral wide-field focusing optic can be produced for arelatively low cost. In an optional variant, this wide-field focusingoptic is no longer multispectral—its spectral sensitivity range islimited to the far infrared—this being operationally conceivable onaccount of the fact that the wide-field configuration is generallylimited to the operation of target detection. The cost of this final,wide-field focusing optic whose sensitivity range is limited to theinfrared band III is even lower.

FIG. 1 shows schematically an example of a multispectral multifieldoptical combination according to the invention in the narrow-fieldconfiguration. The optical axis oa of the optical combination isdepicted by the dot-dash line, the optical combination being a centeredoptical system having substantially a symmetry of revolution about thesaid optical axis oa. Unless otherwise indicated, there is no differencebetween “light”, “light rays” and “light signal”. The path of the lightrays is depicted by arrows directed along the direction of propagationof said light rays. The light rays depicted by dotted arrows representlight rays masked or blocked by elements of the optical combination. Theoptical combination according to the invention comprises a first primarymirror M1, a secondary primary mirror M5, a secondary mirror M2, a firsttransport mirror M3 and a second transport mirror M4, the transportmirrors M3 and M4 constituting the transport optic. Since the opticalcombination according to the invention is of the Cassegrain type, the“primary” mirror and “secondary” mirror naming is given by analogy withthe conventional Cassegrain setup, depending on the function of themirrors in question. The optical combination has an image focal planelocated just downstream of the secondary mirror M2, that is to say theintermediate image focal plane IFP.

The optical combination has another image focal plane located downstreamof the transport optic, that is to say the exit image focal plane EFP.For reasons of simplicity and legibility of FIG. 1, a single exit imagefocal plane EFP has been depicted; the usual imaging devices includespectral splitting elements that are not depicted in FIG. 1 for the samereasons, but which would be located between the first transport mirrorM3 and the various exit image focal planes.

The first primary mirror M1 is concave, its external diameter being De1,and is holed in its central part, its internal diameter being Di1; it isstructured and placed in such a way that, in the narrow-fieldconfiguration (as in FIG. 1), it reflects the light rays arrivingapproximately parallel on its reflecting face, that is to say the lightrays emanating from an external scene considered as being located atinfinity (rays arriving perfectly parallel could emanate only from ascene truly located at infinity), in the direction of the secondarymirror M2.

The second primary mirror M5 is concave, its external diameter beingDe5, and is holed in its central part, its internal diameter being Di5;it is structured and placed in such a way that, in the medium-fieldconfiguration (see FIG. 2), it reflects the light rays arrivingapproximately parallel on its reflecting face in the direction of thesecondary mirror M2 in a manner similar to the first primary mirror M1,while blocking the light rays reflected by the first primary mirror M1in the direction of the secondary mirror M2; in the narrow-fieldconfiguration (as in FIG. 1), it is offset so as to not block the lightrays reflected by the first primary mirror M1 in the direction of thesecondary mirror M2. The internal diameter Di5 of the second primarymirror M5 is smaller than the internal diameter Di1 of the first primarymirror M1.

The secondary mirror M2 is convex, its external diameter being De2, andis holed in its central part, internal diameter being Di2; it isstructured and placed in such a way that, in the narrow-fieldconfiguration (as in FIG. 1), it reflects the light rays arriving on itsreflecting face emanating from the first primary mirror M1 (emanatingfrom the second primary mirror M5 in the case of FIG. 2 that depicts amedium-field configuration) in the direction of the first transportmirror M3. Between the secondary mirror M2 and the first transportmirror M3, the light rays are focused in the intermediate image focalplane IFP in order to form an image of the external scene considered asbeing located at infinity.

The first transport mirror M3 is concave and holed in its central part;it is structured and placed in such a way that, in the narrow-fieldconfiguration (as in FIG. 1), it reflects the light rays arriving on itsreflecting face emanating from the secondary mirror M2 in the directionof the second transport mirror M4.

The second transport mirror M4 is holed in its central part; it isstructured and placed in such a way that, in the narrow-fieldconfiguration (as in FIG. 1), it reflects the light rays arriving on itsreflecting face emanating from the first transport mirror M3 so thatsaid light rays will be focused in the exit image focal plane EFP toform therein an image of the external scene considered as being locatedat infinity.

The distance dM2IFP represents the distance that separates the holedcentral part of the secondary mirror M2 from the intermediate imagefocal plane IFP. The intermediate image focal plane IFP and the exitimage focal plane EFP are fixed relative to the body and to thestructure of the optical combination, whatever the field configurationadopted. Likewise, the first primary mirror M1, the first transportmirror M3 and the second transport mirror M4 are fixed whatever thefield configuration adopted. In contrast, the second primary mirror M5and the secondary mirror M2 can move relative to the body of the opticalcombination and therefore relative to the intermediate image focal planeIFP.

FIG. 2 shows schematically an example of a multispectral multifieldoptical combination according to the invention in a medium-fieldconfiguration. All the mirrors and image focal planes occupy the samepositions as in FIG. 1, that is to say the same positions as in thenarrow-field configuration, except for the second primary mirror M5which has undergone a linear translation in the direction of thetransport optic so as, on the one hand, to block the light rays (indotted lines) emanating from the first primary mirror M1 and, on theother hand, to reflect the light rays arriving approximately parallel onits reflecting surface in the direction of the secondary mirror M2.

FIG. 3 shows schematically an example of an embodiment of amultispectral multifield optical combination according to the inventionin a wide-field configuration. All the mirrors and image focal planesoccupy the same positions as in FIG. 2, that is to say the samepositions as in the medium-field configuration, except for the secondarymirror M2 and a wide-field focusing optic WFFO that was not involved inthe path of the light rays focused in the image focal planes IFP and EFPand was therefore not depicted in the preceding FIGS. 1 and 2 for thesake of clarity. The integral combination consisting of the secondarymirror M2 and the wide-field focusing optic WFFO has undergone a lineartranslation in the direction of the transport optic so that, on the onehand, the secondary mirror M2 can no longer focus, in the intermediatefocal plane IFP, the light rays reflected by the first primary mirror M1or by the second primary mirror M5 and, on the other hand, the lightrays arriving approximately parallel on the wide-field focusing opticWFFO are focused in the intermediate image focal plane IFP and can thenbe focused in the exit image focal plane EFP by means of the transportoptic. The wide-field focusing optic WFFO has a diameter D3 that is lessthan the internal diameter De2 of the secondary mirror M2 and a focallength f3 which is shorter than the distance dM2IFP depicted in FIG. 1.

FIG. 4 shows schematically an example of another embodiment of amultispectral multifield optical combination according to the inventionin a wide-field configuration. All the mirrors and image focal planesoccupy the same positions as in FIG. 2, that is to say the samepositions as in the medium-field configuration, except for the secondarymirror M2 and a wide-field focusing optic WFFO which was not involved inthe path of the light rays focused in the image focal planes IFP and EFPand was therefore not depicted in the preceding FIGS. 1 and 2 forreasons of clarity. The integral assembly formed by the secondary mirrorM2 and by the wide-field focusing optic WFFO is integrally mounted on astructure ST shown symbolically by a dotted square. The structure ST canrotate relative to an axis of rotation AR. The weights, not depictedhere in FIG. 4 for reasons of clarity, are integrally mounted on thestructure ST so that the structure ST remains balanced throughout itsrotation movement about the axis of rotation AR. The structure ST hasundergone a rotation about the axis of rotation AR so that, on the onehand, the secondary mirror M2 can no longer focus, in the intermediatefocal plane IFP, the light rays reflected by the first primary mirror M1or by the second primary mirror M5 and, on the other hand, the lightrays arriving approximately parallel on the wide-field focusing opticWFFO are focused in the intermediate image focal plane IFP and can thenbe focused in the exit image focal plane EFP by means of the transportoptic. The wide-field focusing optic WFFO has a diameter D3 that issmaller than the internal diameter De2 of the secondary mirror M2 and afocal length f3 that is less than the distance dM2IFP depicted in FIG.1.

1. A cassegrain-type multifield optical combination having an imagefocal plane, comprising: a first holed concave primary mirror that isfixed relative to the image focal plane and a convex secondary mirror,the structure and the position, in narrow-field configuration, of themirrors being such that the light that emanates from an external sceneconsidered as being located at infinity and is intended to be focused inthe image focal plane is meanwhile reflected firstly by the firstprimary mirror and then by the secondary mirror, which also includes asecond holed concave primary mirror that can move relative to the imagefocal plane, the structure and the position, in medium-fieldconfiguration, since the medium field is wider than the narrow field, ofthe mirrors being such that the light that emanates from an externalscene considered as being located at infinity and is intended to befocused in the image focal plane is meanwhile reflected firstly by thesecond primary mirror and then by the secondary mirror, wherein thesecondary mirror can move relative to the image focal plane, awide-field focusing optic (WFFO) having a focal length smaller than adistance between the second holed concave primary mirror and the imagefocal plane that can move relative to the image focal planesimultaneously with said secondary mirror, the structure and theposition, in wide-field configuration, since the wide field is largerthan the medium field, of the mirrors and of the wide-field focusingoptic being such that the light that emanates from an external sceneconsidered as being located at infinity and is intended to be focused inthe image focal plane by means of the wide-field focusing optic is notmeanwhile reflected by any of said primary or secondary mirrors.
 2. Theoptical combination as claimed in claim 1, wherein the image focal planeis an intermediate image focal plane, wherein the optical combinationalso has an exit image focal plane, wherein the optical combinationincludes a transport optic between the intermediate image focal planeand the exit image focal plane, and wherein the transport optic consistsof two holed transport mirrors that are fixed relative to theintermediate image focal plane, which lie between the intermediate imagefocal plane and the exit image focal plane and are structured and placedso that the light that emanates from an image located in theintermediate image focal plane and is intended to form an image locatedin the exit image focal plane is meanwhile firstly reflected by thetransport mirror closest to the exit image focal plane, since saidmirror is concave, and then reflected by the transport mirror closest tothe intermediate image focal plane.
 3. The optical combination asclaimed in claim 2, wherein the exit pupil of the optical combination islocated near the transport mirror closest to the intermediate imagefocal plane.
 4. The optical combination as claimed in claim 1, whereinthe wide-field focusing optic is purely refractive.
 5. The opticalcombination as claimed in claim 4, wherein the wide-field focusing optichas a short focal length and a small diameter, its focal length beingshorter than the distance between, on the one hand, the central part ofthe secondary mirror in narrow-field or medium-field configuration and,on the other hand, the image focal plane closest to the secondarymirror, its diameter being smaller than the external diameter of thesecondary mirror.
 6. The optical combination as claimed in claim 1,wherein the wide-field focusing optic is a multispectral optic, thespectral sensitivity range of which extends from the visible into thefar infrared.
 7. The optical combination as claimed in claim 1, whereinthe optical combination includes a rotation system that brings thewide-field focusing optic into position while retracting the secondarymirror.
 8. The optical combination as claimed in claim 7, wherein thatthe secondary mirror and the wide-field focusing optic are mounted on astructure which can rotate and includes one or more weights tocompensate for the imbalance.
 9. The optical combination as claimed inclaim 1, wherein the secondary minor is holed in its central part and inthat the optical combination includes a system for the simultaneouslinear translation of the secondary mirror and of the wide-fieldfocusing optic.
 10. The optical combination as claimed in claim 9,wherein the secondary mirror and the wide-field focusing optic areintegral.
 11. The optical combination as claimed in claim 9, wherein thesecondary mirror consists of a metallization of the peripheral part ofthe rear face of the final optical element of the wide-field focusingoptic.
 12. The imaging device comprising an input optic, comprising anoptical combination as claimed in claim
 1. 13. The imaging device asclaimed in claim 12, wherein the optical combination belongs to theinput optic of one or more light-signal receive channels.
 14. Theimaging device as claimed in claim 13, wherein the number of receivechannels is at least three, these having, as spectral sensitivity range,the near infrared, the mid-infrared and the far infrared, respectively.15. The method of claim 1, wherein the wide-field focusing optic (WFFO)and the secondary mirror are integral.
 16. The method of claim 1,wherein the wide-field focusing optic is multispectral and its diameteris less than that of the hole in the second holed primary concavemirror.
 17. A method of changing field in a cassegrain-type multifieldoptical combination comprising a narrow-field primary concave mirror, amedium-field concave primary mirror and a convex secondary mirror, theprimary mirrors being holed, when passing from a narrow-fieldconfiguration to a medium-field configuration, comprising the steps of:a step of moving the medium-field primary mirror so that themedium-field primary mirror blocks the light reflected by thenarrow-field primary mirror in the direction of the secondary mirror,during passage from a medium-field configuration to a wide-fieldconfiguration, a step of moving the secondary mirror and of moving awide-field focusing optic simultaneously with said secondary mirror, sothat the wide-field focusing optic focuses the light coming from theoutside without it being reflected by one of the primary or secondarymirrors of the optical combination the wide-field focusing optic havinga focal length smaller then a distance between the secondary holedmirror.
 18. The method of changing field as claimed in claim 17, whereinthe wide-field focusing optic (WFFO) is purely refractive.
 19. Themethod of changing field as claimed in claim 18 wherein, since thesecondary mirror is holed in its central part, the step of moving thesecondary mirror and the wide-field focusing optic consists of asimultaneous linear translation of the secondary mirror and of thewide-field focusing optic.
 20. The method of changing field as claimedin claim 18, wherein the step of moving the secondary mirror and thewide-field focusing optic consists of a rotation that brings thewide-field focusing optic into position while retracting the secondarymirror.
 21. The method of changing field as claimed in claim 17 wherein,since the secondary mirror is holed in its central part, the step ofmoving the secondary mirror and the wide-field focusing optic consistsof a simultaneous linear translation of the secondary mirror and of thewide-field focusing optic.
 22. The method of changing field as claimedin claim 17 wherein the step of moving the secondary mirror and thewide-field focusing optic consists of a rotation that brings thewide-field focusing optic into position while retracting the secondarymirror.